Arrangement for increasing the tuning range of inductance tuned resonance circuits



Aug. 16, 1949. vLADlMlR 2,479,438

ARRANGEMENT FOR INCREASING THE TUNING RANGE OF INDUCTANGE TUNED RESONANCE CIRCUITS Filed NOV. 12, 1947 lnvefitorz Leonard O.\/ladimi1";

HisAttorney Patented Aug. 16, 1949 {L onard Vladimir, Bridgeport, Conn, 'ass n to General Electric c mp y, a corporation of New York Application November 12, 1947, Serial No. mazes My invention relates to iron core tuning arrangements, and more particularly, to such arningements as are employed "for-tuning resonant circuits such as those used in the local oscillator of a superheterodynereceiver. It, is a primary object ofmy invention to provide an improved apparatus for tuning resonant circuits.

In general, .a parallel resonant circuit may be tuned by varying either the capacitance or the inductance associated with such, a circuit. In some applications, inductance tuning'has been found superior to capacitance tuning in that more precise tracking isobtainable and objectional howl or squeal appearing in theyoutput is eliminated. It is common practice, therefore,

to employ a movable iron core oo-operating with an inductive coil as means for inductance tuning of parallel resonant circuits.

In the oscillator circuit of a :super'heterodyne frequency modulation receiver, for example, it is desirable to employ a parallel resonant circuit which is tunable over a relatively broad band of frequencies, such as the range from 88 to 108 megacycles per second. .It has been extremely .diflicult to achieve a sufiicient variation in inductance with an iron core to permit inductance tuning over such a wide range. It .is aparticular object of my invention to provide means for increasing the tuning range of an inductancetuned parallel resonant circuit.

, Another object of mysinvention is to provide a tuning arrangement, .for a parallel resonant circuit, which obtains the advantages of inductancetuning over an increased band of frequencies.

A more specific object oitmy invention is to provide an iron core arrangement which may be employed in tuning: the locai-oscillatorcircuit of a superhet'erodyne frequency modulation receiver over a frequency range of 80to'1'08'megacycles per second. a 1

g The features of my invention which I believe to be novel are set-forth with particularity in the appended claims. My invention itself, however, together with further objects and advantages thereof, may best be understood by reference to the following description, taken in connection with the accompanying drawing, in which Fig. .1 is a schematic circuit diagram illustrating the operation of my invention; Fig. 2- is an equivalent circuit of the circuit of Fig. 1;,Fig. 3 is a diagram, in schematic form, of an oscillator circuit which suitably embodies my invention; and Fig. 4 is a view-of a coilwhich may used in the circuit of Fig. 1. Like indicia signify like elements in the several figures.

'2 Glaims. (cram-40) Refer-ring to Fig. laIparaIlelresohant circuit 1 "is shown comprising a capacitance C1 and an inductance L1. The input terminals 2, 3 are "con cult I. An adjustable tuning core, represented nected to a suitable source of radio frequency energy, not shown. ,A second inductance L2 is wound on the same form. with the inductance L1,. and is-connectd in parallel with a capacitance C2. Theinductance 1e and capacitance CS2 constitute a second parallel resonant circuit 4 which is mutually coupled with the first resonant cirschematically at 5, serves to tune simultaneously both resonant circuits I, 4. In addition, the mutual inductance between the two parallel resonant circuits 1, 4 is found to vary with adjustment of the iron core 5, since insertion er the core eflectively decreases the reluctance and obtains increased flux linkage. Iltisunderstood, then, that the movable iron. core effects sii'nultanjeous adjustments of the values of the separate inductances L; and Lz, and ofthe mutual inductance, hereinafter designated M, between them. The

second circuit 4 reflects anefiective impedance Zm,.not shown, to thefirst circuit l by virtue of the mutual inductance M. The values of induc- Referring to Fig. 2, the circuitof Fig. 1 is shown ,in equivalent form in. order to facilitate the analysis of the operationof my invention. The

effective reflected impedance Zm is shown in general form ,and in series with the inductance Li.

magnitude and sign of the reflected impedanceIZm is given by the relation where 'ais the customary phase shift operator,

M is the mutual inductance between L1 and In, 1

w isthe angular frequency of the energy supplied to the input terminals 2, 3, and Z5 is the inipedance ,of the secondarycircuit. If a high Q obtains, the resistances involvedv are negligible in comparison with the reactances Xc and X0 of the coil L2 and condenser C2, respectively, and

where X! is the total reactance of the secondary parallel resonant circuit.

i-llnce:.;thesecondary circuit is designed; to ;al

( XL: X62 at the primary resonant frequency ,1. Hence, XS,

the reactance ofthe series circuit combination of inductance L2 and capacitance C2, is always of a capacitive nature, 1. e., is always associated with a negative 7' operator. Thus, letting a l al fj s and combining with Equation 1,

Equation 6 shows the reflected impedance'to be I always inductive in nature.

It will be noted, from Expression 1, thatthe magnitude of the reflectedlimpedance 'Zm is di rectly proportional to the squares of the angular frequency w and the mutual inductance M. As the iron core enters the tuning coil, the value of inductance L1 increases, and the resonant angular frequency w of the parallel circuit I (Fig. l) decreases. Atthe same time, the mutual inductance M increases; furthermore, the rate of increase of the mutual inductance M is greater than the rate of decrease of the angular frequency .w. As a result, the quantity (wM), and hence the reflected impedance Zm,' increases as the ironcore is inserted in the tuning coil. The net result is an effective increase in the inductance .of the coil L1, due to the reflected impedance Zm frornthe secondary circuit, which provides an increased tuning range.

In. operating the circuit of Fig. 1 it was found that the frequency tuning range of a parallel circuit, such as the circuit I was increased more than 7% .by adding a suitable secondary circuit, such as the circuit 4. V

a Fig. 3 is a schematic diagram of a circuit which is essentially that of a Colpitts oscillator modified to embody my invention. An electron discharge device 6, having cathode, control grid, and anode electrodes I, 8, 9, respectively, is provided. Positive direct energizing voltage +B is supplied to the anode 9, through a suitable inductance choke Ii. The cathode I is connectedto ground (.-B) through a radio frequency choke ID. A modified form of the circuit of Fig- 1 is employed as the oscillatory circuit, terminal 2 being connected to the grid 8 of the device 6, and terminal 3 being directly'connected to ground. The catpacitance C1 is divided into two series capacitances I2, I3 andthe cathode I of the device 6 is directly connected to the junction point- I4 'between the capacitances I2, I3. The anode circuit is completed by means of a blocking condenser l5 connected between the anode!) of the device 6 and ground. Another blockingcondenser I5 is provided in the grid circuit in order to isolate the bias voltage developed across resistance H from the oscillatory circuit I. By these means,the device 6 is made to serve as a "generator of oscillatory energy, the frequency .of the output being dependent on the resonant frequency'of the parallel circuit I.

In this application, the frequency output of the oscillator isvariable over a certain tuning range 'byrelative positioning of the magnetic core 5 in the form (not shown) on which both the in- *ductances-lLiyLz' are mounted; The use ofthe .4 secondary circuit 4 provides an increased tunin range, as set forth above.

Referring to Fig. 4, there is shown the aforementioned coil form I9 on which the inductances L1, L2 are wound. The inductances coils L1, L2 are most conveniently wound in a bifilar manner, the turns of the secondary coil L2 being wound between those of the primary coil L1. I have secondary coil comprises a somewhat smaller number of turns than the primary coil. The coil construction- -is.-not critical, however, and the actual frequency range extension is determined chiefly by tracking'consideration. The form I9 as shownissubstantially cylindrical in form, and is provided with a-longitudinal cylindrical hole I8. The magnetic core 5 formed of compressed powdered iron particles is also substantially cylindricalin shape, and is adapted to be variably positioned in the hole I8. By these means variations in-the self-inductancesiln, L2 are affected simultaneously with variations in the mutual inductance M between them, as heretofore described.

While I haveshown' and described a particular embodiment of my invention,.it will be understood that numerous modifications and variations may be made withoutdeparting fromthe true spirit'and scope of my invention. 1 .What I claim as new and desire to secure by Letters Patent of the United-States is:

1. In combination, a resonant circuit comprising an inductance: andz'a capacitance connected inparallel therewith, an" iron core movable within saidinductance for tuning lsaid circuit over a range of frequencies, and'm'eans for increasing the range of frequency over. which said; circuit-is tunable comprising a secondinductance" coupled to said first inductance and a second capacitance connected'in parallel 'with'said second inductance, said second inductance and capacitance being tuned to a frequency greater than the frequency to which said circuit is tuned, and. the resonant frequency of said'secondlinductance. and capacitance being varied with movement of said iron core. 7

2. In a radio receiver. arranged to receive waves variable over a desired bandof frequencies'a first tuned circuit comprising an inductance and a capacitance connected in: parallel therewith, an iron core movable within said inductance to vary the tuning of said resonant circuit over a band of frequencies less'than said desired band of frequencies, and means for increasing the band of frequencies over which said circuit is tunable comprising a second resonant circuit comprising'a second inductance coupled to said 'first inductance and a second. capacitance connected in parallel with said second. inductance, said second resonant circuit being tuned to a frequency higher than the frequency of said :first circuit, and the value of said second inductance being variable with movement of said iron core.

LEONARD O. VLADIMIR.

REFERENCES CITED The following references are of record in the file of this patent: 

